Polymers hitherto mainly used as optical materials are polymethyl methacrylate and polycarbonate. However, the former has a problem of rather high water absorption and the latter has a problem of development of birefringence in injection molding. Accordingly, it is becoming difficult for these polymers to meet recent requirements, which are becoming increasingly more severe.
In recent years, polymers which use polycyclic norbornene or allied monomers have been developed as polymeric materials improved in said properties in question. For example, Japanese Patent Application Kokai (Laid-Open) No. 60-26024 describes that the hydrogenated products of ring-opening polymers of tetracyclododecenes or of ring-opening copolymers of tetracyclododecenes and norbornenes are excellent in transparency, water resistance and thermal resistance.
However, the hydrogenated products of ring-opening polymers of tetracyclododecenes are not fully satisfactory in moldability and their birefringence values are not so small as can meet the property requirement sufficiently. The hydrogenated products of ring-opening copolymers of tetracyclododecenes and norbornenes also have similar problems. Although hydrogenated products of this kind having a high proportion of norbornenes in copolymerization of 40-50% by mole are somewhat improved in said properties, they have a low glass transition temperature (Tg) of 90.degree.-100.degree. C. and hence their thermal resistance is not fully satisfactory.
Japanese Patent Application Kokoku (Post-Exam. Publn.) No. 58-43412 describes that the hydrogenated product of dicyclopentadiene ring-opening polymer can be easily heat-melt processed to give a tough, transparent sheet. The hydrogenated product, however, is unsatisfactory in thermal resistance for use as optical disks because it has a low glass transition temperature (Tg) of about 95.degree. C.
On the other hand, polymers or copolymers obtained by using polycyclic norbornene or allied monomers, when they have not been hydrogenated, are poor in resistance to oxidative degradation and hence unsuitable for use as optical materials.